This invention pertains to the field of examination, material characterization and imaging of the internal structure of a body by acoustic techniques. An important application of this field of technology is in diagnostic medicine, wherein safe, accurate and noninvasive examination techniques are becoming increasingly important. X-ray and gamma ray techniques have been used for many years to obtain imagery of the internal structure of the body. Since the dangers of radiation dosage buildup have become better understood, the use of such techniques has been limited to those instances in which the risk represented by the additional X or gamma ray radiation dosage is outweighed by the need for diagnostic information.
However, in other fields which involve examination of areas of the body that are highly susceptible to radiation damage or are statistically prone to form cancers, alternatives to X-ray or gamma ray radiation have been sought. This is particularly true where examination is required on a routine and repeated basis, such as breast examination and in the fields of obstetrics and gynecology.
Promising developments have been made in the field of ultrasonic examination and imaging in order to overcome these problems. Although it is known that very high energy levels of ultrasonic energy are harmful to the human body, low energy levels of exposure are not known to have any harmful effects. Unlike the case with X-ray or gamma ray radiation, there appears to be no reciprocity law (of damage equalizing the product of beam power and exposure time) concerning exposure to ultrasound, so long as the doses are kept below a threshold level. Fortunately, workable signal levels fall well below the danger point. Ultrasonic scanning and imaging devices thus hold the promise of permitting noninvasive examination of internal body structures on a repeated and routine basis, without any presently known, nor suspected, harmful side effects.
Although previously developed ultrasound systems have provided useful information for physicians, they have left considerable room for improvement in terms of resolution, repeatability of measurement, and type and quality of data obtained.
One type of prior art ultrasound device is known as the A-scan which is widely used in various medical fields, including echo cardiography, gynecology and in measuring the position of the brain center line. While not actually providing an image of the tissue, the A-scan provides a display such as a cathode ray tube (CRT) with one axis representing time (depth of penetration), and the other axis showing echoes or return pulses.
Another type of ultrasonic device known as the B-scan is used in obstetrics and cardiology. The B-scan also uses an echo mode, with the transducer scanned laterally along the area of interest of the body. The lateral position of the transducer is displayed on the X-axis, with the depth or distance of the echo being displayed on the Y-axis, and the strength or amplitude of the return echo modulating the brightness of the display. The B-scan is subject to a number of disadvantages resulting from its use of the pulse-echo mode. The echoes or back scatter from some delicate tissue structures may be too weak to be received, particularly since the reflected energy is further attenuated on travelling back through the tissue to the receiver.
Another disadvantage of the B-scan is that the strength of a return from a surface within the body is not only a function of the physical properties of the surface, but also a function of its angular relationship to the pulses. Thus a portion of the surface which happens to be at nearly right angles to the beam gives a strong echo, while another portion of the same surface at a different angle gives a weak return. This same effect makes it difficult or impossible to accurately calibrate B-scan apparatus in order to give consistent and repeatable results, from one clinical setting to another. Slight differences in positioning of the transducer with respect to the patient will give differing brightness levels for the same structures within the body. This lack of consistency is due to the fact that the B-scan does not obtain a quantitative measure of an intrinsic tissue property, but rather measures a property which is an interaction between the measurement system and the body. Recent B-scan methods, known as Compound B-scanning, have employed superimposition of B-scans taken from a number of different angles in an attempt to overcome the above-noted limitations, but such techniques per se still do not measure an intrinsic property of the tissue.
Another prior art acoustic imaging system known as the C-scan involves forming a two dimensional projection for representation of the body at right angles to the beam. In some systems, energy transmitted through the body is received on the other side, and its amplitude serves as a measure of the attenuation in the body. In some recent work, lensing has been employed in an attempt to focus on a single plane within the body, but resolution and contrast are generally poor, and it is not possible to measure an intrinsic property of the tissue at a point in such systems.
In an echo C-scan, range gating is used to select a desired image plane in the body, but the systems are subject to intense specular reflections making it very difficult or impossible to obtain structural data.
Other workers have proposed the use of acoustic holography to produce attenuation-type two dimensional projections of a body similar to the C-scan system. However, the present lack of sensitivity of such systems requires high input acoustic energy levels and therefore raises a possible question of harmful effects. Such systems to date have produced acoustic attenuation C-scan images of the reflection and transmission with their attendant limitations as detailed earlier.
Another acoustic imaging method developed by R. C. Heyser and D. H. LeCroisette is reported in the IEEE 1973 Ultrasonics Symposium Proceedings (IEEE Catolog No. 73CHO 807-8SU). This method involves transmitting swept frequency bursts through a body to a receiver on the opposite side. The received signal is beat against the original signal, giving a frequency difference due to the time delay of propagation. Variations in the difference frequency are then converted to a voltage and painted on a CRT, to give a fringe contour plot which is essentially another form of two dimensional projection.
The above prior art systems, although promising in many respects and very useful in some applications, have not been able to provide quantitative data of intrinsic properties of the internal structure of a body, nor have they been able to provide maps or images along sections passing through the body. This latter type of data and imagery are obtainable in the X-ray field by means of computerized tomography, tomographic reconstruction, etc. and systems commonly called scanners. Unfortunately, however, earlier attempts to apply the same techniques using ultrasonic energy have produced poor results.
In the case of X-ray scanners, the body is exposed to a plurality of beams of radiation through a number of different directions in a plane through which the section is to be imaged, and the amplitude of the received radiation is measured. With enough sets of data to define a matrix of small picture elements in the body, algebraic reconstruction techinques are used to solve for the attenuation coefficient of each picture element. These data can then be displayed graphically by means of a CRT.
We have previously proposed a system which uses an analogous method, with a source of ultrasonic pulses and a receiver or an array of receivers on the other side of the body for receiving beams of ultrasonic energy transmitted through the body. See J. F. Greenleaf, S. A. Johnson et. al., Acoustic Holography, Vol. 5, Plenum Press, New York, 1974, pp. 591-603. In that system the amplitude of the received pulses are measured, and an algebraic reconstruction technique is employed to calculate the attenuation coefficient of the various picture elements. The resolution and accuracy of such systems are limited by refraction of the beams within different tissue structures, and to a greater degree by reflection of energy both at the surface of the body and from tissue structure within the body. Since the amount of reflection loss from a given beam is unknown and unpredictable, there is no practical way to discriminate between the amplitude loss in a received signal due to attenuation losses within the body, and the losses due to reflection within the body. This imposes a fundamental limitation upon the resolution of the system, and upon the accuracy of the attenuation coefficient being calculated.
We have discovered that these limitations of acoustic reconstruction measurement and imagery can be avoided by measuring the time-of-flight of acoustic energy pulses through the body, rather than measuring amplitude losses. Mathematical reconstruction techniques are then employed to calculate the spatial distribution of acoustic velocities throughout the plane of measurement in the body. Since only the time of arrival of a beam is detected, and not its amplitude, the system is immune to reflection problems.
The velocity data represents actual measurement of intrinsic properties of the tissue being scanned, and has diagnostic value because different types of tissue, both normal and abnormal, have characteristic acoustic propagation velocities or velocity ranges. Further, the data can be displayed by means of a CRT to give an image or map of values along the section.
The velocity values thus obtained can be used in conjunction with attenuation values to achieve a synergestic effect. The combination of attenuation coefficient data and acoustic velocity data (or index of refraction data) for each picture element in the section gives the diagnostician far greater information than from the sets of data individually.
The index of refraction data obtained on a first calculation can then be used to recalculate the velocity data, or attenuation data, to take into account the bending of ray paths through the body due to the changes in index or refraction from point to point, so as to achieve a higher degree of resolution.